An aromatic polyimide film has a high chemical stability, a high physical property, a high electronic insulating property, and a high heat resistance. Therefore, the aromatic polyimide film has been employed in a wide variety of arts. Particularly, the heat resistant aromatic polyimide film is favorably used as a support for an electroconductive sheet in the form of a laminate of an aromatic polyimide film, an adhesive layer and an electroconductive metal sheet such as a lead frame of an electronic part.
An aromatic polyimide film is prepared in the form of a continuous longitudinal film having a large width, while a lead frame of an electronic part generally is a part of a small size having precise dimensions and a punched area with complicated design. Accordingly, the aromatic polyimide film should be cut sharply and precisely with no defective section when the polyimide film is cut or punched.
GB 2,278,497-A (corresponding to U.S. Pat. No. 5,593,774) describes that an insulating polyimide film should have an edge tearing resistance of 50 to 70 kgf/20 mm so that the polyimide film can be cut with little occurrence of burrs and chips. It is further described that the polyimide film should contain a certain amount of a residual solvent so as to give the desired polyimide film having the designated edge tearing resistance.
The present inventors have studied various kinds of aromatic polyimide films and found that the polyimide films described in the above GB patent publication do not give a highly heat-resistant aromatic polyimide film showing both a satisfactory cutting or punching property and a good adhesion to an adhesive layer.